a) Field of the Invention
The present invention relates to a flange back mechanism for a photographic camera, particularly applicable to a lens-interchangeable-type single-lens reflex camera wherein the focus adjustment is performed by adjusting the back focus.
b) Description of Related Art
In typical single-lens reflex cameras, the focus adjustment with respect to each distance to a subject is effected by a distance ring of the lens.
Single-lens reflex cameras in which the distance ring is automatically driven to a focusing position by a motor incorporated in the body or a motor incorporated in the lens have been put to commercial use as AF single-lens reflex cameras, but these AF single-lens reflex cameras require AF lenses, and thus it has been impossible to perform the autofocus function with conventional MF lenses.
In addition, conventional single-lens reflex cameras adopt a fixed-type flange back mechanism, so that a closer shot than the shortest photographing distance peculiar to the lens has been impossible.
To overcome this problem, one of the present inventors has proposed single-lens reflex cameras in which a camera body is comprised of a body-front fixed section, which mainly consists of a driving power supply, a driving mechanism, a camera-holding grip having a release button, and the like, using as a reference a lens mount allowing a conventional lens to be mounted and interchanged, and a back movable section which mainly consists of a distance-measuring mechanism including a distance-measuring element, a film-exposing mechanism, a viewfinder, and the like, whereby focus adjustment is made by an amount of movement of the back movable section relative to the body-front fixed section (U.S. patent applicatoin Ser. No. 08/226,004).
According to the techniques disclosed therein, it is possible to move the shortest photographing distance of conventional lenses to a further closer photographing side, and to achieve the autofocus function with an automatic drive system.
In this case, the position of a flange back, which serves as a reference which is common to various types of lenses that are mounted on the camera, is very important. This position is used as a reference position of autofocus adjustment with the lens be set at infinity.
However, since the conventional MF lens is not provided with a locking mechanism such as the one provided in a lens exclusively used for AF, the conventional MF lens can be moved undesirebly even in a case where the back-drive type system is set to an autofocus mode. That is to say, AF response with the MF lens has been insuffcient if the lens is not set in the position of infinity or the lens is offset slightly therefrom even though the photographer might have thought that it has been set at that position.
Accordingly, there has been the problem that the photographer must pay heed to the position of the distance ring of the lens at all times so that the distance ring of the lens is surely set at the position of infinity.